1. Field of the Invention
The present invention relates to the design of Ethernet passive optical networks. More specifically, the present invention relates to a method and apparatus for reducing data burst overhead in an Ethernet passive optical network.
2 Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies that are presently being developed, Ethernet passive optical networks (EPONs) are one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically performed between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which is typically an external network belonging to an Internet Service Provider (ISP) or a local exchange carrier. An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is typically coupled to the OLT through a single optical link. (Note that one may use a number of cascaded optical splitters/couplers.) This configuration can save significantly in the number of fibers and amount of hardware required by EPONs.
Communications within an EPON can be divided into downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, downstream data frames are broadcast by the OLT to all ONUs and are subsequently extracted by their destination ONUs. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler with the OLT.
Correspondingly, an EPON typically employs some arbitration mechanism to avoid data collision and to provide fair sharing of the upstream fiber-channel capacity. This is achieved by allocating a transmission timeslot to each ONU. An ONU typically buffers data it receives from a subscriber until it reaches the start time of its transmission timeslot. When its turn arrives, the ONU “bursts” all stored frames to the OLT at full channel speed.
Due to unequal distances between an OLT and ONUs, optical signal attenuation in an EPON is not the same for each ONU. The power level received at the OLT could be different for each transmission timeslot. This is called the near-far problem. If the receiver in the OLT is adjusted to receive a high-power signal from a closely located ONU, it may mistakenly read a “one” as a “zero” when receiving a weaker signal from a distant ONU. Similarly, if the receiver is adjusted to a weak signal, it may read a “zero” as a “one” when receiving a stronger signal. To detect an incoming signal properly, the OLT receiver is ideally given a short period to adjust its zero-one threshold, which is called the automatic gain control (AGC) period, at the beginning of each timeslot. In addition, another period is usually reserved after the AGC period for the receiver to synchronize its clock with the incoming bits. A clock and data recovery (CDR) circuit is responsible for the bit-synchronization.
Another issue is that it is not enough just to disallow an ONU from sending data outside its assigned transmission timeslot. Even in the absence of data transmission, an ONU's laser generates spontaneous emission noise when powered on. Accumulated spontaneous emission noise from several ONUs close to the OLT can easily obscure the signal from a distant ONU (this is called the capture effect). Thus, an ONU ideally shuts down its laser between its transmission timeslots. Because a laser takes time to cool down when turned off, and to warm up when turned on, its emitted power may fluctuate at the beginning and the end of a transmission. Therefore, a laser turn-on period and a laser turn-off period are typically reserved for the laser to stabilize.
During the laser turn-on, turn-off, AGC, and CDR periods an ONU cannot transmit payload data. This data burst overhead makes the upstream bandwidth utilization less efficient. Hence, what is needed is a method and apparatus for reducing data burst overhead in an Ethernet passive optical network.